Organic light-emitting diodes (OLEDs) make use of the property of materials whereby they emitting light when suitable charge carriers are formed by applying a voltage. Recombination of these charge carriers forms excited states, which in turn enter the ground state by emitting light. OLEDs are an interesting alternative to cathode ray tubes and liquid crystal displays because they are suitable for production of flatscreen displays and displays for mobile applications, such as cellular phones, notebooks, PDAs, etc., due to their very compact design and their low power consumption.
To improve the efficiency of organic light-emitting diodes, they often have charge transport layers in addition to the actual emission layer, these transport layers being responsible for the transport of negative and positive charge carriers into the emission layer. These charge transport layers are divided into hole conductors and electron conductors, depending on the type of charge carriers transported.
Organic light-emitting diodes (OLEDs) usually consist of various layers of organic materials, at least one layer (emission layer) containing an electroluminescent substance which can be made to emit light by applying a voltage (Tang, U.S. Pat. No. 4,769,292). High-efficiency OLEDs are described in U.S. Pat. No. 7,074,500, for example.
Organic solar cells are known from the prior art, for example, US 2009217980 and US 2009235971. Organic solar cells comprise a layered stack on a substrate, such that the layered stack has at least one organic light-absorbing layer which is arranged between two electrodes (anode and cathode). At least one electrode must be transparent in the wavelength range in which the solar cell should function, typically in the strong absorption bands of the absorption layer in the visible and near-infrared ranges.
The absorption layer of a solar cell may be formed by a donor-acceptor heterojunction. This heterojunction may be a shallow junction, such that donors and acceptors are formed in adjacent layers (optionally with an intermediate layer). The heterojunction may also form a volume heterojunction such that donors and acceptors are mixed in the same layer.
Solar cells may also be stacked such that the at least two absorption layers are electrically linked by a pn-junction (also known as a recombination or connection unit). Such pn-junctions are known, for example, from US 2009045728 and EP 2045843.
Solar cells preferably also have organic hole and electron semiconductor layers which are essentially transparent. These are used for optical optimization, but they do not contribute toward the absorption. In addition all of these transport layers are preferably doped.
Doped OLEDs and doped solar cells as well as tandem solar cells are known from Walzer et al., Chem. Rev. 2007, 107, pages 1233-1271, for example.
Organic components are components containing at least one organic semiconductor layer. The organic semiconductor layers contain so-called “small molecules” among other organic molecules or also organic polymers, such that the organic molecules and the organic polymers either as a single layer or as a mixture with other organic materials (for example, described in US2005 0110009) or inorganic materials have semiconductor properties or metal-like properties.
As semiconductor components which constitute a high percentage of inorganic semiconductor elements and/or layers and at the same time contain one or more organic semiconductor layers or organic semiconductor materials or so-called organic-inorganic hybrid components. Within the context of the present invention, these hybrid components are also to be understood as organic components.
Components such as organic light-emitting diodes have a high potential for applications in the field of lighting and displays in particular in one embodiment for generating white light. In recent years, definite improvements have been achieved in this field with regard to the efficiency achieved as well as with regard to the lifetime of the components. The power efficiencies of stable white OLEDs today are in the range of 10 to 50 lm/W, and lifetimes of more than 10,000 hours can be implemented. For a broadly conceived commercialization in the field of general lighting applications, however, further improvements will be needed in particular with regard to the power efficiency because the market for highly efficient technologies for generating white light, for example, fluorescent tubes, is dominated today by efficiencies of up to 100 lm/W, for example.
Typical organic light-emitting diodes have the disadvantage that only approximately 25% of the light generated is emitted from the component. Approximately 50% of the light remains as internal modes in the arrangement of organic layers between the two electrodes. An additional 20% is lost in the substrate due to total reflection in the substrate. The reason for this is that the light is formed with a refractive index of approximately 1.6 to 1.8 within an OLED in optical media. If this light then strikes an optical medium with a lower refractive index, for example, another layer within an OLED stack, the substrate on which the OLED is formed or one of the electrodes, then complete reflection occurs if a certain value of the angle of incidence is exceeded.
For the use of white OLEDs in lighting technology it is also necessary to employ suitable output methods which can also be tied into the manufacturing process inexpensively. It is assumed today that an area of an OLED of 1 cm2 for lighting applications may cost only a few cents if its application is to be economically reasonable. However, this also means that only especially inexpensive methods can be used at all to increase the light output. OLEDs based on so-called small molecules are processed today with the help of thermal evaporation in vacuo. OLEDs typically consist of two to twenty layers, all of which are vapor-deposited individually thermally. If it is possible now to significantly improve the output with the help of only a single additional thermally deposited layer, then at any rate the condition is fulfilled in the costs of the output method.
For applications of OLEDs as lighting elements, it is also necessary to design the components over a large area. For example, if an OLED is operated at a brightness of 1000 cd/m2, then areas in the range of several square meters would be necessary to illuminate an office room, for example.
According to the state of the art, benzidine derivatives such as N,N′-diphenyl-N,N′-bis(3-methylphenyl)-1,1′-biphenyl-4,4′-diamine (TPD), 4,4′-di-(N-carbazolyl)-diphenyl (CBP) and N,N′-di-(alpha-naphthyl)-N,N′-diphenyl-1,1′-biphenyl-4,4′-diamine (alpha-NPD) have been used in the past as hole-conducting materials and/or electron-blocking materials, but they lack thermal stability, which depends essentially on the glass transition temperature of these compounds (TPD: Tg=65° C.).
The lower thermal stability may result in destruction of the layered arrangement or mixing of different molecules from different layers, so that over time under thermal stress, an electronic or optoelectronic component will lose its efficiency.
Materials with polycyclic aromatic structures, for example, anthracene, pentacene, tetracene, phthalocyanines and also materials such as the fullerene C60 are used for components because of the high charge carrier mobility for organic materials. For example, thin-film field effect transistors (OTFT) may be mentioned here.
Through conventional methods such as vacuum vapor deposition, such components having multiple layers of an organic and/or polymer material can be produced. The relatively high crystallinity of the layers is partially also responsible for the high charge carrier mobility. However, layers of these molecules have the disadvantage of crystallization of the layer which can thus make the component unstable. Larger molecules, for example, sexithiophenes cannot be vaporized successfully without great decomposition.
The fullerenes, mainly C60 here, have a great n-charge carrier mobility and are also used as active semiconductor layers in OTFTs. However, C60 is very susceptible to contamination by oxygen, for example, and the components must be encapsulated at great expense.
In addition to the aforementioned organic materials there are hardly any organic materials which, at room temperature or higher temperatures, have a charge carrier mobility great enough to produce efficient OTFTs, for example, from them by the usual production methods. Other methods for efficient production of OTFTs are far less complicated and also require improvement (for example, change in the dielectrics of the surface, change in the dielectrics of the materials, charge carrier injection layers).
A high charge carrier mobility is desired to minimize the losses due to space charge effects in OLEDs or OPVs and also to increase the maximum usable frequency of digital or analog circuits as well as vibrating circuits (oscillators). New materials that exhibit a high conductivity with stable dopants are fundamentally desired.
A method of minimizing the power loss in organic components due to space charge effects is the use of doped layers. Doping increases the conductivity of the layer and thus bypasses the problem of the low charge carrier mobility.
It is known that organic semiconductors may be modified in their electrical properties, in particular in their electrical conductivity, by doping, as is the case with inorganic semiconductors (silicon semiconductors). By creating charge carriers in the matrix material, an increase in the initially quite low conductivity as well as a change in the Fermi level of the semiconductor are achieved, depending on the type of dopant used. Doping here leads to an increase in the conductivity of charge transport layers so that ohmic losses are reduced and an improved transition of the charge carriers between contact and organic layer is achieved. Doping is characterized by a charge transfer from the dopant to a nearby matrix molecule (n doping, electron conductivity increased) and/or by the transfer of an electron from a matrix molecule to a nearby dopant (p doping, hole conductivity increased). The charge transfer may be complete or incomplete and can be determined, for example, by the interpretation of the vibration bands from FT-IR measurements.
The conductivity of a thin-film sample can be measured using the so-called two-point method in which contacts made of a conductive material, for example, gold or indium-tin oxide, are applied to a substrate. Next, the thin film to be tested is applied to the substrate over a large area, so that the contacts are covered by the thin film. The current then flowing is measured after applying a voltage to the contacts. From the geometry of the contacts and the layer thickness of the sample, the conductivity of the thin-film material is obtained from the resistance thereby determined.
At the operating temperature of a component having a doped layer, the conductivity of the doped layer should exceed the conductivity of the undoped layer. To do so, the conductivity of the doped layer should be high at room temperature, in particular greater than 1×10-8 S/cm, but preferably in the range between 10-6 S/cm and 10-5 S/cm. Undoped layers have conductivities of less than 1×10-8 S/cm, usually less than 1×10-10 S/cm.
The thermal stability can be determined using the same method and/or the same structure by heating the layer (doped or undoped) in steps and measuring the conductivity after a resting time. The maximum temperature, which the layer can withstand without losing the desired semiconductor property, is then the temperature immediately before the conductivity collapses. For example, a doped layer on a substrate with two electrodes side by side as described above may be heated in increments of 1° C., waiting 10 seconds after each increment. Then the conductivity is measured. The conductivity changes with the temperature and drops abruptly above a certain temperature. The thermal stability therefore indicates the temperature up to which the conductivity does not collapse abruptly.
In these methods it is important to be sure that the matrix materials have a sufficiently high purity. Such purity can be achieved with traditional methods, preferably by gradient sublimation.
The properties of the various materials involved can be described through the energy layers of the lowest unoccupied molecular orbital (abbreviated: LUMO; synonym: electron affinity) and of the highest occupied molecular orbital (abbreviated: HOMO; synonym: ionization potential).
One method of determining ionization potentials (IP) is ultraviolet photoelectron spectroscopy (UPS). As a rule, ionization potentials are determined for the solid state but it is also possible to measure ionization potentials in the gas phase. The two values differ due to solid-state effects, for example, the polarization energy of the holes occurring in the photoionization process (N. Sato et al., J. Chem. Soc. Faraday Trans. 2, 77, 1621 (1981)). A typical value for the polarization energy is approximately 1 eV, but greater deviations may also occur.
The ionization potential is based on the start of the photoemission spectrum in the range of the high kinetic energies of the photoelectrons, i.e., the energies of the photoelectrons having the weakest bonding.
An associated method, namely inverted photoelectron spectroscopy (IPES) can be used to determine electron affinities (EA). This method is not very widespread however. Alternatively, solid-state energy levels may also be determined by electrochemical measurement of oxidation potentials (Eox) and reduction potentials (Ered) in solution. Cytovoltammetry (CV) is a suitable method. Empirical methods of deriving the solid-state ionization potential from an electrochemical oxidation potential are described in the literature (for example, B. W. Andrade et al., Org. Electron. 6, 11 (2005); J. Amer. Chem. Soc. 127, (2005), 7227.).
No empirical formulas are known for converting reduction potentials to electron affinities. This is due to the difficulty in determining electron affinities. Therefore a simple rule is often used: IP=4.8 eV+e·Eox (vs. ferrocene/ferrocenium) and/or EA=4.8 eV+e·Ered (vs. ferrocene/ferrocenium) (cf B. W. Andrade, Org. Electron. 6, 11 (2005) and refs. 25-28 therein). For the case when other reference electrodes or redox pairs are used for referencing the electrochemical potentials, conversion methods are known (cf. A. J. Bard, L. R. Faulkner, “Electrochemical Methods: Fundamentals and Applications,” Wiley, 2nd edition, 2000). Information about influencing a solvent can be found in N. G. Connelly et al., Chem. Rev. 96, 877 (1996).
It is customary although not actually accurate to use the terms “energy of the HOMO” E(HOMO) and/or the “energy of the LUMO” E(LUMO) as synonymous with the terms ionization energy and/or electron affinity (Koopmans' theorem). It should be pointed out that the ionization potentials and electron affinities are such that a higher value means a stronger binding of a released or added electron. The energy scale of the molecular orbitals (HOMO, LUMO) runs in the opposite direction. Therefore in rough approximation it holds that: IP=−E(HOMO) and EA=−E(LUMO).
WO 2007/118799 describes quinoid heteroacene materials as organic semiconductors. JP 2002 124384 A2 describes 12-diazapentathanes and derivatives thereof. Diazapentathanes are also disclosed in US 2003 099865 A.
U.S. Pat. No. 6,242,115 B1 relates to organic light-emitting diodes in which asymmetrical charge transport materials having tertiary amine functions are used. These tertiary amine functions are constructed from the biphenyl core and two additional phenyl groups which are not joined together but may also be linked together directly or via a saturated or unsaturated bridge. A saturated and/or unsaturated C2 bridge is proposed as such a linkage but it must not have any additional substituents.
In the state of the art the lack of thermal stability, the charge carrier mobility and the atmospheric stability of the hole-conducting and/or electron-blocking compounds used so far in electronic, optoelectronic and electroluminescent components have represented a technical shortcoming which has limited the use of these components.